The present invention generally relates to an optical repeater, and more particularly to an optical repeater that prevents malfunctions due to the presence of noise.
In optical communication systems, optical repeaters are provided in an optical fiber cable at predetermined intervals so that the occurrence of a receipt error due to the length of the optical fiber cable can be prevented. On the other hand, each optical repeater also serves as a noise source. Thus, there is also an error which takes place during signal processing of each optical repeater. Such signal processing errors increase with an increase in the number of optical repeaters. From this point of view, each optical repeater is equipped with a supervisory circuit, which supervises structural elements of the optical repeater and outputs supervisory signals indicating the operating state of each structural element.
Referring to FIG. 1, there is illustrated a conventional optical repeater. The configuration shown in FIG. 1 is provided in an up line, and the same configuration is provided in a down line. An avalanche photodiode (hereafter simply referred to as an APD) 11 receives an optical input signal sent from a terminal or an adjacent optical repeater (not shown) via the up line, and converts the optical input signal into an electrical signal. An equalizer 12 equalizes the electrical signal having a distortion by feeding back a control signal to the APD 11. That is, the APD 11 and the equalizer 12 form a full AGC (automatic gain control) loop. Although not illustrated for the sake of simplicity, normally, an electrical AGC utilizing the gain of an amplifier is provided. A timing generator 14 extracts a timing signal from the equalized electrical signal output by the equalizer 12. A decision circuit 13 identifies the level of the equalized electrical signal at a timing defined by the timing signal output by the timing generator 14 and generates a reshaped digital signal. A high-speed interface circuit (hereafter simply referred to as an interface circuit) 15 sends the reshaped digital signal to a bandpass filter (BPF) 3 and adds a supervisory signal produced and outputted by a low-speed supervisory device 40 to the reshaped digital signal that is to be transmitted. The bandpass filter 3 extracts (demodulates) a command signal in analog form from the digital signal supplied from the interface circuit 15 and sends the same to the low-speed supervisory device 40. The low-speed supervisory device (hereafter simply referred to as a supervisory device) 40 executes a procedure instructed by the extracted command. For example, the supervisory device 40 checks each structural element and sends the aforementioned supervisory signal indicative of the check results to the interface circuit 15.
The digital signal with the supervisory signal added thereto is sent to regeneration circuits 16 and 18, which respectively drive laser diodes (LD) 17 and 19 in accordance with the digital signal from the interface circuit 15. The regeneration circuit 16 serves as a normal-use circuit, and the regeneration circuit 18 serves as an emergency-use circuit. It is also possible to alter the relationship between the regeneration circuits 16 and 18 by a signal which is generated by the supervisory circuit 40 in response to a command. An optical coupler 20 passes through either the optical signal from the laser diode 17 or the optical signal from the laser diode 19. An optical receive/decision circuit 1 is made up of the APD 11, the equalizer 12, the decision circuit 13, the timing generator 14 and the interface circuit 15.
FIG. 2 illustrates a detailed structure of the supervisory device 40 shown in FIG. 1. The supervisory device 40 shown in FIG. 2 comprises a command detection circuit 5, a command switch 7 and a supervisory circuit 8. The command detection circuit 5 comprises an amplifier 51, a feedback resistor 52 connected between input and output terminals of the amplifier 51 and a comparator 53. The amplifier 51 amplifies the command signal extracted by the bandpass filter 3 (FIG. 1). The comparator 53 compares the level of the amplified command signal with a threshold level Lth, and generates a command detection signal indicative of the presence/absence of the command signal. The command switch 7 selects either the command signal output by the amplifier 51 of the command detection circuit 5 or a command signal output by a command detection circuit (not shown) related to the other (down) line in accordance with the command detection signal generated and outputted by the command detection circuit 5. The selected command signal is sent to the supervisory circuit 8.
Referring to FIG. 3, a command detection circuit 5a, a command switch 7a and a supervisory circuit 8a are related to the up line and correspond to the above-mentioned command detection circuit 5, the command switch 7 and the supervisory circuit 8, respectively. On the other hand, a command detection circuit 5b, a command switch 7b and a supervisory circuit 8b are provided with respect to the down line.
A description will now be given of the operation of the configuration shown in FIG. 3 with reference to FIGS. 4A through 4D.
Referring to FIG. 4A, when the command detection circuit 5a detects the command signal supplied from the bandpass filter 3 for the up line while no command signal related to the down line is detected, the command detection circuit 5a controls the command switch 7a so that it selects the command signal output from the command detection circuit 5a related to the up line. The command signal sent from the command detection circuit 5a is fed to the supervisory circuit 8a.
As shown in FIG. 4B, the command detection circuit 5b detects the command signal from the bandpass filter related on the down line while no command signal is detected by the command detection circuit 5a, the command detection circuit 5b controls the command switch 7b so as to select the command signal from the command detection circuit 5b. Normally, each of the command switches 7a and 7b selects the command signal supplied from the other line. Thus, the command switch 7a related to the up line selects the command signal output by the command detection circuit 5b related to the down line.
Referring to FIG. 4C, when the command switch 7a is supplied with the command signals from both the command detection circuits 5a and 5b, the command switch 7a related to the up line preferentially selects the command signal from the command detection circuit 5a related to the up line.
In the above-mentioned switching operation of the command switches 7a and 7b, the supervisory circuits 8a and 8b are supplied with the command signals and collect information about the operation of each structural element related to the up and down lines. For example, the supervisory circuit 8a checks the structural elements related to the up line and generates the supervisory signal indicating the check results, which is sent to an optical repeater coupled to the up line.
However, the aforementioned conventional optical repeater has the following disadvantage. Even when no optical input is applied to the APD 11 shown in FIG. 1, a dark current passes through the APD 11 because of the AGC control. Thus, the equalizer 12 outputs a signal (noise) having a substantially fixed amplitude level based on the dark current. In many cases, the level of the noise signal at the output of the amplifier 51 of the command detection circuit 5 is higher than the threshold level Lth (FIG. 2). In the case shown in FIG. 4C, the comparator 53 (FIG. 2) of the command detection circuit 5a detects the noise signal as if it detects the command signal on the up line. Thus, as shown in FIG. 4D, the command detection circuit 5a controls the command switch 7a so that it selects the command signal from the command detection circuit 5a irrespective of the absence of the command signal from the up line. As a result of this switching, the supervisory circuit 8a cannot supervise the structural elements in response to the command signal from the other (down) line.